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Liquifying Oxygen

Note: I was not able to get any more liquid nitrogen to make a video version of this demonstration. The entire change takes only about 30 seconds and the attraction to the magnet is easily observable.

What You Do...

A test tube and balloon full of oxygen gas are suspended from the ceiling. The test tube is carefully lowered into a Dewar flask of liquid nitrogen.

What You See...

The balloon shrinks as the gas cools. The balloon also becomes brittle looking. When you lower the flask away from the test tube, a pale blue liquid is visible in the tube... liquid oxygen. The tube immediately frosts up, so you need to carefully wipe away the frost from the VERY cold test tube.

You can demonstrate the fact that oxygen is paramagnetic by placing a strong magnet near the tube and drawing it away. The liquid in the tube will be slightly attracted to the magnet. If you repeatedly pull the tube with the magnet at the correct frequency, you can make the tube swing significantly.

The Set Up...

I saw Bob Becker (great demonstrator... clever guy) show an easy way to liquify methane. This lecture experiment is adapted from that demonstration. We are able to get liquid nitrogen from our local state college. We have small Dewar flasks just the size to fit a test tube. We hang a test tube from the ceiling on a string. Next we flush out the tube with oxygen (we have a tank of oxygen in the room... from the local welding supply), fill a balloon and place the balloon over the top of the test tube. Hang the tube from the ceiling.



There is a discussion of why liquid oxygen is blue in Bassam Shakhashiri's demonstration book.[1]

This lecture experiment also provides a demonstration of the Gaw Laws (as the temperature lowers, the volume and pressure of the gas decreases) as well as intermolecular forces of attraction (the fact that the nonpolar molecule, O2, is so difficult to liquify signifies a very small attraction between the molecules... London dispersion forces.

Additionally, the fact that O2 is paramagnetic is the classic demonstration of the usefulness of the molecular orbital theory... Paramagnetism depends on a substance having unpaired electrons. Conventional bonding theory shows O2 as two atoms sharing two pair of electrons in overlapping orbitals... a double bond.

For most of first-year chemistry, this model is adequate and very useful. However, if O2 is paramagnetic, where are the unpaired electrons? Enter... molecular orbital theory...

Molecular orbital theory involves the idea that the orbitals surrounding each of the oxygen atoms (the 1s, 2s, and 2p orbitals) interact as the two nuclei come near each other... the result is a new set of orbitals that belong to the two nuclei together... the molecular orbitals. When these orbitals are filled (see the orbitals inside the center box), the two highest energy electrons are unpaired... making the molecule paramagnetic.

Safety and Disposal...

Oxygen is an oxidizer. The test tube must be clean so no oxidizable materials (fuels) come in contact with the liquid oxygen. The liquid nitrogen and liquid oxygen are also very cold so they should not be touched. Disposal is easy... I just pour it on the floor.

[1] Shakhishiri, B.Z. "Chemical Demonstrations Volume 1--A Handbook for Teachers of Chemistry"; The University of Wisconsin Press: Madison, Wisconsin, 1983

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